In motion picture production, even though the bulk of image capture is done on film media, a considerable amount of editing and post-production work is carried out using digital image manipulation tools, a process known as the digital intermediate process. During this digital intermediate process, a key process in preparing a motion picture production, images obtained during filming or generated digitally are displayed on a display device, a specific example of which is an electronic projector. In order for the display device to display the image, the display device image data must be sent to the display device. This display device image data is a set of code values that define the color to be displayed at each pixel. Based on these code values, the display device emits light that produces the proper color on the viewing screen. It is noted that because the digital intermediate process involves modifying the image so that the intent of the creative person can be shown, the display device image data may have no direct relationship to the film image and no direct relationship to the original scene. In the case of computer-generated imagery, such as cartoons, there never was an original scene. The final approval of the motion picture may be made by viewing a projected film print or by viewing the motion picture projected with an electronic projector. The motion picture shown to the paying public is now commonly projected using either a film projector with a film print of the motion picture or an electronic projector with a digital version of the motion picture.
When images are projected in a dark theater, people adapt to the overall luminance of the light reflected from the screen. After adaptation, it is impossible for a person to accurately estimate the luminance of a projected white patch. This is the reason that projected images can look real even though the luminance on the screen is a small percentage of the luminance of the original scene. For example, a white object in direct sunlight may have a luminance of 5000 nits and a black object in the same scene may have a luminance of 50 nits. A white object in a motion picture projected in a dark theater may have a luminance of 50 nits and a black object in the same scene may have a luminance of 0.025 nits. But both white objects will be judged to be white and both black objects will be judged to be black by observers adapted to those viewing conditions. Because of luminance adaptation, a person cannot judge the overall luminance of a scene or an object.
A visual phenomenon to which people do not adapt well is colorfulness. Colorfulness is defined herein using the definition from Digital Color Management Encoding Solutions by Giorgianni and Madden, page 541, “Colorfulness—Attribute of a visual sensation according to which an area appears to exhibit more or less of its hue.” The same object, for example a piece of clothing, will appear very colorful when in direct sunlight, will appear considerably less colorful when in the shade or on a cloudy day, and even less colorful when indoors in a dimly lit room, and will appear to have no colorfulness if viewed when illuminated by starlight only. This phenomenon has been described in detail by Hunt in the articles R. W. G. Hunt, Journal of the Optical Society of America, 42, 190-199 (1952); R. W. G. Hunt, Journal of the Optical Society of America, 43, 479-484 (1953); R. W. G. Hunt, Color Research and Application, 2, 55-68 (1977); and R. W. G. Hunt, Color Research and Application, 2, 109-120 (1977). People tend to remember and prefer the high level of colorfulness that is associated with high levels of luminance such as outdoor sunlight conditions. The decrease in colorfulness due to a decrease in the luminance of the object is generally not preferred and is considered inferior to the sunlight conditions. Likewise, a motion picture that was created by viewing the images at a certain luminance level will look inferior if the motion picture is projected at a lower luminance level, not because the change in luminance is so visible, because it is not, but because the loss in colorfulness is visible when the luminance is decreased.
So that the paying public can see very high quality motion pictures independent of the theater that is entered and so that the creators of the motion pictures can be assured that the paying public is seeing their art displayed as they intended the images to be seen, the Society of Motion Picture and Television (SMPTE) has written standards and recommended practices that define the viewing conditions for viewing motion pictures in a dark theater. One such standard is SMPTE 431-1, which defines the white luminance that is to be measured off the screen in a dark theater when the screen is illuminated by an electronic projector. This white luminance is standardized to be 48 nits. However, due to decreased light output from a projector bulb as the bulb ages, it is common for the white luminance reflected from the screen to be below the standardized 48nits. In this case, the paying public may not notice the decrease in luminance, but they will see an inferior image due to the loss in colorfulness of all colored objects in the motion picture. Accordingly, it would be advantageous if when the white luminance is low, the low colorfulness of the colored objects in the motion picture could be brought back to the same colorfulness as when the motion picture was projected at the standardized white luminance. Alternatively, instead of bringing the colorfulness back to the colorfulness with the standardized white luminance, the colorfulness could be altered toward the colorfulness with the standardized white luminance even if it is not brought back completely to the colorfulness with the standardized white luminance. This will still give an improved or higher quality image relative to no change in colorfulness.